Ramsey Electronics SHA2 - Manual (Page 7)

SHA2 • 7
The power supply
VR1, C29 and C28 supply the regulated DC voltage for the circuit. VR1
regulates the input, which should be around 12VDC input to J5, to a level of
10VDC for use in the unit. Yes the internal supply voltage indicates it is 9VDC
and here is the reason why. You will notice that diode D1 is connected
between VR1 and the power switch. This diode is to prevent voltage being
applied to the output of VR1 from battery BAT1. Diodes are neat things
because they will only conduct current in one direction such as in this case
when a positive voltage is applied to the anode of the diode with respect to the
cathode. (OK… the cathode is the side of diode to which the arrow points in
the schematic or the end with a band or ‘mark’ on it in the actual device. The
anode is the other side. The cathode likes to be negative with respect to the
anode.) Anyway, you never get something for nothing and in the case of the
diode when it is conducting there is about .6 volts lost across it so the voltage
supplied to the circuitry is around 9.4 volts but we’ll just call it 9VDC.
You will notice that the power jack, J5, has a switch contact which disconnects
the battery from the circuit when external power is being used. This prevents
the battery from attempting to charge when you are using an external power
supply. It is not a good idea to attempt to charge a battery, in fact it can cause
a battery to explode, if the battery is not designed to be charged. NEVER
attempt to charge a standard alkaline or carbon/zinc battery.